Purification of liquids



Patented Sept. 20, 1938 PURIFICATION oF LIQUIDS Curtis Q. McW. Campbell, Pittsburgh, Pa.

No Drawing.

Application June 17, 1935,

Serial No. 26,923

4' Claims. (or. 204-125) This invention relates to the clarification of liquids, such as the aqueous wastes and wash waters from various industrial processes, andsewage. It relates especially to the treatment of sewage 5 and other liquids containing organic matter in suspension or emulsion therein, although it is also applicable to the treatment and clarification of many otherkinds of suspensions and emulsions.

The principal object of my invention is to provide an improved process of clarifying waste liquids and other suspensions and emulsions. Another object is to provide an improved coagulant for use in such a process. A further object is to 5 provide a method of purifying liquids containing organic matter. A still further object is to provide a method of clarifying and simultaneously reducing the bacterial content of sewage and the like. Other objects and advantages of my invention will appear from the following description. I. In my copending application, Serial No. 554,-

486, filed August 1, 1931, now Patent No. 2,051.983, I have described an improved coagulant and clarifying material and a method of using such material. This coagulant is prepared by making up an aqueous dispersion of starch containing about 1.5% to 10.0% or more of corn or potato starch, for example. This dispersion is then preferably heated sufficiently to expand the starch cells without rupturin a substantial portion of them, and then allowed to cool. panded starch is then mixed with an alkaline reagent, such as a solution of caustic soda, caustic potash or ammonia, and this produces a coagulant which is useful for many purposes, as described in my Patent No. 1,942,507. In accordance with the procedure described in my pending application, however, the starch-caustic coagulant is then subjected to an electrolytic treatment which improves its properties, and when the electrolytic treatment is used the heating step may in some instances be omitted. The electrolytic treatment has the twofold effect of increasing the coagulative power of the sci or coagulant for most types of particles in suspension or emulsion in aqueous liquids, and also of imparting a marked bactericidal property. to the coagulant.- This bactericidal property is especially marked when the coagulant is subjected to electrolytic treatment in closed vessels, preferably with direct current, immediately before adding it to the liquid to be clarified or treated. Under such circum-- stances the electrolytic' treatmenthas the effect of liberating nascent oxygemwhich is dissolved in or carried along with the coagulant.

The dispersion of ex I have now found that similarly beneficial results are obtained by adding oxygen, and especially nascent oxygen, to the liquid to be treated, with certain reagents or coagulants other than the starch-caustic coagulant previous y described. I have also found that this is particularly beneficial if the nascent oxygen is formed from, or liberated by, or produced by the action of, a constituent of the coagulant under appropriate conditions, as the handling of the materials to obtain the desired results is thereby facilitated. With some compositions electrolytic treatment of the coagulant is necessary to obtain this result, but such is not always the case, and in some instances oxygen may be advantageously supplied from an external source either to replace or to supplement that obtained from the coagulant itself byelectrolysis or otherwise.

Certain salts, such as salts of iron and aluminum, have a known coagulating effect under suitable conditions, and have been used heretofore to assist in clarifying waters and aqueous wastes in certain processes. I now propose to increase the efiectiveness of such materials, as well as others, by suitable treatment prior to use. ment preferably has as its main feature the passage of an electric current through a solution of the selected salt or salts, and I have discovered that a freshly electrolyzed solution apparently has its coagulative power increased whether the current employed is direct or alternating, especially if the solution contains adsorptive or coagulative suspended matter. When it is desired to reduce or eliminate bacterial and other organic matters in addition to suspended solids, however,

it is preferable to subject the solution to a direct current pretreatment, preferably in a vessel or conduit which is c osed to the atmosphere. Water is thereby decomposed, and oxygen is liberated at the positive electrode while hydrogen is liberated at the negative electrode. The hydrogen may be vented to the atmosphere but the nascent oxygen is carried along with the solution, usually at least partially dissolved therein, and is added as promptly as possible to the liquid to be treated. If it is difficult tocorrectly proportion the rate of flow of the solution or other coagulant and the current density and consequent oxygen liberation during the treatment, or if, for some other reason, it is diflicult to get enough oxygen liberated and carried along with the solution to producethe desired effect, additional oxygen may be added from an external source. It is usually most convenient to conduct the electrolysis in a divided chamber or in a bent conduit similar to a U-tube.

This treat- The solution or other coagulant then flows in at one 'side or leg near the top, then down that a side, under the partition or other dividing strucadditional oxygen is being supplied, it can be,

admitted at any convenient point along the path 'of the solution or other coagulant being treated, but it is preferably admitted into or near the outlet or positive side ofthe cell. When the oxygen supply is so connected, the oxygen can be added without interference with the treatment, regard less of whether the treater is operating on alternatin or direct current, or even if it is not operating at all.

If the salt solution or other coagulant subjected I to the electrolytic treatment contains chlorine, in the form of a chloride for example, some of this chlorine is also liberated at the positive terminal of the cell and may be carried along with the coagulant into the liquid to beclarifled or purified. This is advantageous, especially when elimination of bacteria or organic matter is one of the objects of the treatment,- as chlorine is very effective for that purpose. Also, the liberaton of the chlorine makes the remaining solution more alkaline, especially when the chlorine comes from analkali or alkaline earth chloride,

and this is usually desirable as increased alkalinity facilitates the coagulation of suspended matter in, and the clarification 01, many aqueous wastes. For these reasons it is often desirable to add to the coagulants which are to be subjected instances.

to my electrical treatment alkaline chloride which may not in itself have any direct coagulating ef fect, such as sodium chloride, for example.

A modification of the above procedure, which depends largely on the same properties of the reagents for its advantages, is also useful in many In this modification I start with a solution of a suitable alkali or alkaline earth chlorde, preferably sodium chloride, which may have any desired concentration, such as 1% to 20% or more. I have found that about 8% is a convenient concentration, and I will use that concentration for purposes of illustration in the following example. d rect current electrolytic treatment similar to that described hereinabove, and liberates chlorine at the positive electrode and hydrogen at the negative electrode while forming sodium hydroxide in solution. The treatment may be stopped as soon as the solution is saturated with chlorine, or before that time, or it may be conmay also be wasted or it maybe burned or other- ,wse utilized. The resulting alkaline about 5% caustic soda, or more .or less, for ex- This salt solution is subjected to a solution I containing instances, but it is generally desirable to heat the suspension to a temperature of about 40 to 60 C. for a short time (e. g., about 15 to 20 minutes) to expand the starch cells substantially without rupturing them. Higher temperatures up to the boiling point may be employed if desired, but treatments at the higher temperatures should preferably be limited in time to prevent excessive degeneration of the starch. After the heating it is' preferable to dilute the starch to a concentration of about 1% to 3%, although more concentra solutions or suspensions may also be used with good effect. I

The proportions in which the starch and the I alkaline solutions are mixed, as -well as their respective concentrations, may be varied over a rather wide range. For clarifying coal washery water containing 5 to 8% of fine particles of coal, for'example, about 3 parts of a 1% starch dispersion and 4 parts of 3% caustic soda solution for each 2000 parts of water give good results. When treating more acid wastes, a higher proportion of alkaline reagent may be desired, and when treating substantially neutral or alkaline wastes containing very fine suspended or colloidal matter, it may be desirable to increase the proportion of starch with or without reducing the amount of caustic. For any particular application or set of operating conditions the optimum concentrations and proportions are readily determined'by simple tests, but it is generally found that a wide range of concentrations and proportions give satisfactory results. It should be understood that if the mixture of starch dispersion and electrolytically decomposed chloride solution is not suillciehtly alkaline in any given instance, as when the decomposition is not carried far enough or the solution is diluted excessively after the electrolytic treatment, additional alkaline reagent of any desired kind can be added to the mixture before use.

' In many instances it is desirable to subject the coagulative mixture to further electrolytic treat.- ment to increase its activity and to liberate more nascent oxygen and/or chlorine Just before the mixture is added to the liquid to be clarified or purified. When this is done, the initial electrolytic treatment of the salt solution 'may be omitted, and the starch dispersion is then mixed directly with a salt solution which may also I contain any desired amount of alkaline reagent.

. This is usually preferable when the 'costs of electric power and the'sodium chloride or other salt employed are high as compared with the cost of the alkaline reagent, and it may be preferable whenever relatively small amounts of clorine and/or oxygen are required for the satiswhich mixture is produced by any of the above described alternative procedures, is then added to I the liquid to be treated. This may be done by introducing the mixture into a fiume or other conduit, which is preferably provided with baflies or mixing devices, through which the liquid to .be

treated is delivered into a settling tank or' other clarifying receptacle ofsuitabie known form. Alternativ'ely, the mixing fiume may be omitted and 'a simple discharge pipe may be employed, with or without a mixing pump. The impurities in suslQ pension or emulsion in the liquid are rapidly coagulated in the presence of my coagulant, and settle to the bottom of the tank, from which they are withdrawn as sludge. The clarified liquid generally overflows the top of the tank, and is discharged or returned to the process in which it originated or otherwise disposed of. When elimination of bacteria or dissolved organic matter is an essential requirement of the treatment, the chlorine liberated in the salt decompositionstage,

or obtained elsewhere, and/or the oxygen ob- ;tained by electrolysis or otherwise, and/or other material capable of liberating oxygen, such as a hypochlorite, may be supplied to the liquid in the tank in any suitable manner during the coagulation. Alternatively, the sewage or other liquid containing undesired substances in suspension or in emulsion may be treated with the coagulant and thereby clarified as described hereinabove, and

the clarified eiiiuent may then be treated with the chlorine and/or oxygen and/ or other oxygen-liberating compound.

In any case, very effective coagulation and clarification are obtained in a relatively short time,

and the process 'is so flexible that it is-readily adapted to the simultaneousv removal of any required amount of bacterial and organic matter in the most economical manner, regardless of local or varying market conditions.

I claim:

1. The process of treating impure aqueous liquids which comprises forming an aqueous dispersion of starch, mixing the dispersion with an alkaline solution containing a chloride to produce a coagulative sol, passing an electric current through the .sol to liberate oxygen and chlorine,-

adding the sol to saidimpure liquid, and separating purified liquid from substances concentrated by said sol.

2. The process of preparing a bactericidal coagulant, which comprises forming an aqueous so-- lution of sodium chloride, passing an electric current therethrough to decompose the chloride and liberate chlorine while forming an alkaline solution having chlorine dissolved therein, mixing the solution with an aqueous dispersion of expanded starch to form a coagulative sol having bactericidal properties, and subjecting the sole to further electrolytic treatment to improve its coagulative and bactericidal properties.

3. The process of treating sewage and the like,

which comprises forming an aqueous solution of sodium chloride, passing an electric current therethrough to decompose the chloride and liberate chlorine while forming an alkaline solution with chlorine dissolved therein, withdrawing excess chlorine separately from the solution, mixing the solution with an aqueous dispersion of expanded starch to form a coagulative sol having bactericidal properties, mixing the sol with said sewage to be treated, separating clarified liquid from substances concentrated by said sol, and contacting the clarified liquid with the said excess chlorine to further reduce its bacterial content.

4. The process, of preparing a bactericidal coagulent for the treatment of impure'liquids, which comprises forming an aqueous solution containing a chloride-selected from the class consisting of al- CURTIS Q. McW. CAMPBELL. 

